Process of electropolishing zinc



May 7, 1963 Filed Nov. 3, 1958 J. A. M. LE DUC PROCESS OF ELECTROPOLISHING ZINC 5 Sheets-Sheet 1 STEEL, STAINLESS STEEL MAGNETITE HEAT -TREATED 316, HEAT-TREATED (Before) (Before) (Before) W] "H M h hiiamiu' STEEL, STAINLESS STEEL MAGNETITE HEAT-TREATED 3|6,HEATTREATED (After) him |liiii;;ii-

DRUM STEEL STAINLESS STEEL,3I6 STEEL (Before) (Before) RUST ovERso ef e) STMNLESS STEEL, ST

EEL (After) RUST ovsRso (After) FIG. 6

INVENTOR JOSEPH ADRIEN M. LE DUC imp/(14% 5 Guw ATTORNEYS y 1963 J. A. M. LE DUC 3,088,886

PROCESS OF ELECTROPOLISI-IING zmc Filed Nov. 3, 1958 5 Sheets-Sheet 2 CAST IRON PURE ZINC GRAPHITE [Before] (Before) CAST IRON PURE ZINC GRAPHITE (After) (After) (After) 7 F/G. 6 F/G. 9

MAGNESIUM ZAMAC-3 NICKEL-A (Before) (Before) W "III" I hiiz- MAGNESIUM ZAMAC- 3 NICKEL-A (After) 7 (After) (After) F/G. /0 F/G.

INVENTOR JOSEPH ADRIEN M. LE DUC ATTORNEYJ May 7, 1963 .1. A. M. LE DUC 3,088,886

PROCESS OF ELECTROPOLISHING ZINC Filed Nov. 3, 1958 5 Sheets-Sheet 3 PLATINUM TITANlUM ZIRCONIUM (Before) (Before) (Before) Ihi' whim PLAHNUM THANIUM ZiRCONIUM (Amer (After) (AHer) F/ G. /3 FIG. /4

SILVER MOLYBDENUM ALUM\NUM Before Before (Before) 1 H- h \lliini'l ln V I S1LVER MOLYBDENUM ALUMINUM (After) (After) (After) INVENTOR JOSEPH ADRlEN M. LE DUO ATTORNEYS May 7, 1963 J. A. M. LE DUC PROCESS OF ELECTROPOLISHING zmc 5 Sheets-Sheet 5 Filed Nov. 3, 1958 5 6 CELL POTENTIAL (VI ID CTRODE POTENTIAL ICATHODIC) (VI 4 5 CELL POTENTIAL (VI I ELECTRODE POTENTIAL (CATHODIC) IV) LEGEND CURVE MATERIALS A--- PLATINUM B---CAST IRON C---STAINLESS STEEL-HEAT TREATED D-"STEEL -HEAT TREATED E-"MAGNETITE (CAST) F---STEEL IO/IO INVENTOR G-"STAINLESS STEEL 310 I---PURE ZINC SHEET (BRIGHT) J---STEEL SHEET PLATED ZINC fl'M f on! ATTORNEY United States Patent 0 3,088,886 PROCESS OF ELECTROPOLISHING ZINC Joseph Adrien M. Le Duc, Painesville, Ohio, assignor to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware Filed Nov. 3, 1958, Ser. No. 771,507 15 Claims. (Cl. 204-1405) This invention relates to the electropolishing of zinc and zinc alloys and, more particularly, to a process for the anodic polishing of zinc and zinc alloys and to an electrolyte composition for use in such a process.

Zinc and zinc alloys, particularly zinc alloys containing 90% or more zinc, are presently widely used as the base material for making plated articles, particularly chromium plated articles. In such uses, a casting is made of the zinc or zinc alloy, which casting is customarily then trimmed, strapped and boiled. generally in an automatic bufling machine, to smooth the surface and to eliminate, as much as possible, any pits therein. The surface of the casting is then degreased and cleaned, after which it is plated, first with copper, then nickel and finally chromium, to give the finished plated article.

In this procedure, one operation, that of butting, is done by hand and is the most time-consuming and expensive, often accounting for as much as 50% of the total finishing cost. Additionally, because all metals possess some degree of malleability, mechanical polishing causes deforming and crushing of the crystal structure of the metal near its surface. This deformation and crushing adversely affects the friction and wear properties of the metal, as well as reducing the corrosion resistance and endurance strength of the metal, even after plating.

In order to overcome the structural disadvantages which result from mechanical butling. as well as to substantially reduce the time and cost of such an operation, and obtain a deburring as well as briwtening of the workpiece, it has been proposed to polish zinc and zinc alloy castings electrolytically. In such operations, electrolytes comprising chromic and/or phosphoric acid have generally been used. However, these materials are relatively expensive and hence the electropolishing operation using such electrolyte baths, are not appreciably more economical than mechanical buffing. Alkaline electrolytes have also been used, such as those consisting of an alkali metal hydroxide and cyanide or an alkali metal hydroxide alone. However, the former bath, because of the cyanide contained therein, is potentially dangerous to those working with it and hence not desirable. For this reason, electrolytes consisting essentially of alkali metal hydroxides, alone, are preferred.

In view of this, considerable work has been done to determine the operating conditions under which electropolishing takes place in an alkali metal hydroxide electrolyte. As reported in the Canadian Journal of Chemistry, vol. 31, 1953, pages 422 to 438, zinc and zinc alloy castings may be electropolished in an alkali metal hydroxide electrolyte containing from about 200 to 1400 g./liter of alkali metal hydroxide using a current density of from 163 to about 909 amps/square foot at a temperature from 32 to 194 F. Additionally, it has been found to be desirable to incorporate small quantities of zinc oxide in the electrolyte to minimize pitting of the casting. However, although the above demonstrates the feasibility of electropolishing zinc and zinc alloys in an alkali metal hydroxide electrolyte, on a laboratory basis, the described process has not, up to this time been found to be applicable for commercial operations.

The primary cause for the unsatisfactory characteristics of this method is that considerable quantities of spongy zinc are deposited on the cathodes of the electropolishing apparatus during its use. This spongy deposit of zinc on the cathode becomes larger as the electropolishing is continued, loosens itself from the cathode, floats on the electrolyte and becomes mixed therewith. This floating spongy mass comes in contact with the anode work, thereby short-circuiting above the solution, in a hydrogenoxygen atmosphere, often resulting in an explosion.

In an effort to eliminate the formation of this porous zinc deposit, mercury has been added to the electrolyte so as to cause a harder, more adherent zinc deposit to form on the cathode rather than the spongy zinc ordinarily formed. Although, by the use of such expedient, the initial zinc, plated on the cathode, is hard and nonporous, within a short time the deposit again becomes spongy and floats on the electrolyte surface. Similar results are obtained with other additives, such as lead.

An additional cause of unsatisfactory characteristics of this method is that the alkali metal hydroxide electrolyte, as proposed in the above cited Canadian Journal, is subject to an excessive amount of foaming in use, which foaming due to hydrogen evolution and surface tension of the bath, makes polishing of the zinc or zinc alloys extremely diflicult.

It has now been found, in the practice of the present invention, that within the broad ranges of operating conditions, for the anodic electropolishing of zinc and zinc alloys in alkali metal hydroxide electrolytes, as disclosed in the above Canadian Journal, considerably narrower ranges occur, wherein the anodic electropolishing of zinc and zinc alloys may be carried out on a commercial scale. Additionally, it has been found, that where a cathode having a surface containing Fe O is used, in an electropolishing process carried out within these narrower ranges of operation, there is no formation of spongy zinc plate on the cathode, the zinc remaining in solution in the alkali metal hydroxide electrolyte. Alternatively, an oxidizing agent or compound having available oxygen can be added to the electrolyte to prevent the plating of zinc on the cathode. It has been found, that contrary to the prior beliefs, the inclusion of zinc oxide in the alkali metal hydroxide electrolyte is not essential, and additionally, that certain materials, which will prevent the foaming of the alkali metal hydroxide electrolyte during electropolishing, can be included in the electrolyte, without any detrimental effect thereto.

It is, therefore, an object of the present invention to provide a commercially feasible process for the anodic electropolishing of zinc and zinc alloys.

Another object of this invention is to provide such a process, whereby the electropolishing takes place in an alkali metal hydroxide electrolyte.

A further object of this invention is to provide a suitable alkali metal hydroxide electrolyte for use in the above process.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

s,oss,sse

As used in the description of the invention and the claims, the term, alkali metal hydroxide, is meant to refer to the hydroxides of lithium, sodium, potassium, cesium and rubidium. However, because of its loW cost and ready availability, sodium hydroxide is preferred, and for this reason, primary reference will be made hereinafter to sodium hydroxide.

Referring now to the drawings, which are attached hereto and made a part hereof, FIGS. I to XXIV are reproduced from actual photographs of cathodes, of various materials made before and after the cathodes were used in the electropolishing of zinc in alkali metal hydroxide solutions.

FIGS. XXV and XXVI are graphs illustrating the electrode potentials for some of these cathode metals.

In a commercially feasible process for anodically electropolishing zinc and zinc alloys, the method of the present invention envisions the following operative conditions, within the broad range of operating conditions disclosed in the prior art. The concentration of the alkali metal hydroxide, preferably sodium hydroxide, electrolyte, should be within the range of about 450 to 900 g./liter, with the preferred range being 572 to 763 g./liter, i.e., about a to aqueous sodium hydroxide solution. The temperature of the electrolyte may be varied within the range of to 220 F., depending upon the concentration of the electrolyte which is used. At the higher electrolyte concentrations, e.g., 800 to 900 g./1iter, the preferred temperature range is 125 to 162 F. However, with the preferred electrolyte concentration of 572 to 763 g./liter, these high temperatures will not produce the most effective electropolishing of zinc and zinc alloys, while temperatures within the range of to F. give the best electropolishing and hence are the preferred temperatures for this concentration range.

Within the above ranges of electrolyte compositions, at the preferred temperatures given, satisfactory electropolishing of zinc and zinc alloys is carried out over a wide range of current densities, e.g., 50 to 800 amps./ square foot for various periods of time. For example, at the higher current densities, the time required for good electropolishing will. be about 1-2 minutes, while at lower current densities, the time required will increase to 4 minutes or somewhat more. However, it will be appreciated by those skilled in the art, because of the higher electrical load carrying equipment which is required to provide high current densities, which equipment is not standard in most electrolytic apparatus, current densities of about 200 amps/square foot are preferred. At this current density about 4 minutes is required for good electropolishing. Additionally, it has been found, that in the electropolishing process carried out under these conditions, the solution current density, which may be defined as the total applied current divided by the total effective volume of electrolyte, should not be in excess of about 2.5 amps/liter. Where this value is exceeded, the zinc or zinc alloy castings are polished but zinc is plated on the cathode.

It has been found, that even when operating the electropolishing process under the above preferred conditions, a spongy, porous zinc is plated on the cathode, which zinc plate, if deposition is permitted to continue, will eventually cause a short circuit in the cell. It has now also been found, that when operating under the above preferred conditions, this zinc deposit is eliminated by using a cathode having a surface which contains Fe O It is to be recognized that Fe O is frequently expressed as Fe O -FeO and as such designation is included in the present invention. Examples of such material are heat-treated steel, black oxide coated steel, hot-rolled steel, magnetite and any other metal or non-metal on which there is a surface which contains Fe O Additionally, by a cathode having a surface which contains Fe O it is meant to include cast iron, which has likewise been found to exhibit the same desirable qualities as the materials mentioned above. Preferred among such cathodes is one of hot-rolled steel or black oxide coated steel. Such a cathode may be in the form of one or several plates or if desired may be the tank which contains the electrolyte. When such a cathode is used, there is no deposition of a spongy, porous zinc plate thereon, the zinc remains in solution as zincate and considerable quantities of hydrogen gas are discharged at the cathode. For the most effective operations, that is, in order to get the most complete dissolution of the zinc into the solution, the area of the cathode should be of the order of 10-40 times the area of the anode.

Alternatively, to prevent a build-up of zinc on the cathode, an oxidizing agent may be added to the electrolyte. Examples of such materials are perborates, peroxydisulfates, oxychlorides, permanganates, bichromates, chlorates, bromates, iodates, nitrates, peroxides, and chlorine. Particularly good results have been obtained using the alkali metal perborates, peroxydisulfates, peroxides and nitrates and the alkaline earth metal oxychlorides, specifically, sodium nitrate, sodium perborate, potassium peroxydisulfate, and sodium peroxide and calcium oxychloride. Although, addition of these materials, in the mount of about 2025 gins/liter of electrolyte is effective in avoiding plating at the cathode, these agents must be replenished about once every eight hours. Because of this, the preferred method of preventing a build-up of zinc on the cathode is the use of a cathode having a surface which contains Fe O While the oxidizing agent in the electrolyte avoids plating of zinc at the cathode the exact mechanism by which the Fe O containing surface on the cathode prevents zinc plating is not known. It is believed, however, that the hydrogen overvoltage value of the Fe O coated cathode has greatly changed the characteristics of the system, so that the current does not plate zinc on the cathode, but rather electrolyzes the contents of the electrolyte, hydrogen being given off at the cathode and the oxygen combining with the zinc and the alkali to form a zincate, which gradually builds up in concentration during electropolishing.

While previously, it was believed to be essential that the alkali metal hydroxide electrolyte, such as sodium hydroxide, contain zinc oxide when the electropolishing begins, in order to prevent pitting of the zinc or zinc alloys which are being polished, it is also known that the addition of zinc oxide to the electrolyte is undesirable, in that as the electropolishing continues, there is a gradual build-up of zinc oxide in the electrolyte until a point is reached at which electropolishing no longer takes place. This point, that is, the concentration of zinc oxide which can be tolerated in the electrolyte, has been found to be about g./liter.

Thus, it can be seen that when zinc oxide is initially added to the electrolyte, the effective electropolishing of the electrolyte, prior to the time replenishment is required, will be shortened. It has now been found, that in a commercial operation, wherein the electropolishing conditions are within the preferred ranges set forth above, although the presence of zinc oxide in the electrolyte does not adversely affect electropolishing, it can likewise be eliminated from the initial bath composition without deletcriously affecting the electropolishing of zinc or zinc alloys, since it accumulates during the electropolishing operation. In actual practice, an electrolyte bath consisting essentially of a 40 to 50% solution of sodium hydroxide may be used for a period of about 2 to 3 weeks before the zinc oxide concentration therein builds-up to the toleration point of 175 g./liter, whereupon the entire bath may be renewed or any portion thereof may be replenished, which is necessary to reduce the zinc oxide concentration to below the toleration point. Alternatively, the bath may be dragged out every day either by removal or loss of solution and the caustic soda electrolyte replenished by the addition of about 5%, by volume of the total bath, caustic soda, thereby maintaining the zinc oxide concentration in the bath below the toleration point for a period of 6 months or more, after which time the entire bath, generally, will have to be replaced.

Additionally, it has been found to be desirable, although not essential, to add to the above sodium hydroxide electrolyte solution a small amount, e,g., .003- .03% by weight, of a material which acts as a defoaming agent. Examples of materials which may be so used are the following:

Anisole Dimethyl octynediol Phenol Cocoyl surcosine Cresol Laurie isopropanolamide Mono-butyl naphthalene sodium sulfonate Di-butyl naphthalene sodium sulfonate Nonyl phenoxy polyoxyethylene ethanol Disodium-N-octadecylsulfosuccinamate Aliphatic substituted butynediols Aliphatic substituted octynediols Aliphatic substituted octynediols mixed with an alkyl phenyl ether of polyethylene glycol in ethylene glycol Alkyl benzyl polyethylene glycol ether Polyoxyethylene esters of mixed fatty and rosin acids Polyethylene tridecyl alcohol Polyethylene tridecyl alcohol and urea Branched chain alcohol ethers Alkyl phenyl polyethylene glycol ether Polyalkylene glycol ethers Polyoxyethylene ester Compounds formed by addition of propylene oxide to ethylene-diamine followed by addition of ethylene oxide Of these, cresol, aliphatic substituted octyncdiols, aliphatic substituted octynediols mixed with an alkyl phenyl ether of polyethylene glycol in ethylene glycol, alkyl benzyl polyethylene glycol ether and polyoxyethylene esters of mixed fatty and rosin acids are preferred.

Where the caustic electrolyte solution is used without a defoaming agent, there is a considerable frothing and foam formation in the electrolyte, which foam, by adhering to the anode and cathode, makes electropolishing of the work pieces extremely difficult. It will be appreciated that this foam may be removed by mechanical means or by use of overflow and recirculation tanks. This, however, is -cumbersome and adds additional steps, as well as cost, to the electropolishing operation, so that the inhibition of the foam by the addition of a defoaming agent, is greatly to be preferred.

Referring now to the half tones which form the illustrative figures in the present application, FIG. I to FIG. III and FIG. VII are Fe O surfaced cathodes. The before and after pictures show that there is no zinc plating where such cathodes are used. In contrast, FIG. IV to FIG. VI and FIG. VIII to FIG. XXIV show that where the cathode surface does not contain Fe t) there is a considerable build-up of zinc on the cathode. The conditions under which electropolishing was carried out with these various cathodes will be shown in the specific examples which are included hereinafter.

In actual operation, an open tank is made of hot-rolled steel or heat treated steel, which tank serves as the cathode. For every ampere of current to be applied, 0.665 liter of an aqueous caustic soda solution, containing from 572 to 763 g./liter of caustic soda are placed in the cathode tank. This quantity of electrolyte gives a solution density not in excess of 2.5 amps/liter. The temperature of this solution is maintained within the range of 70 to 90 F. To this solution is added one of the defoaming agents disclosed above, in the amount of .2 g./liter. The size of the cathode tank is about 20-25 times that of the workpiece anode, i.e., for an anode workpiece having an area of 1 square foot, the cathode tank has an area of 20-25 square feet. The work piece of zinc or zinc alloy which is the anode is placed in the electrolyte solution and using a current of 200 ampcres for each square foot of workpiece area, thereby giving a current density of 200 amperes/square foot, electropolishing is carried out for a period of about 4 minutes. At the expiration of this time, the workpiece is removed from the electrolyte and is found to have a smooth, bright, lustrous surface, free of pitting. Additionally, it is noted, that the cathode tank is completely free of any zinc plate or spongy deposit and there is little if any foam or froth in the electrolyte, thus showing the efiectiveness of the defoaming agent as well as the cathode having a surface containing Fe O in preventing the plating of zinc.

In order that those skilled in the art may better understand the method of the present invention and the manner in which it may be practiced, the following specific examples are given.

In the following examples, unless otherwise specified, the electropolishing is carried out in a 2000 ml. glass beaker, equipped with cooling coils and a hot plate. A 6" x 12" sheet of heat treated steel is formed into a cylindrical cathode and placed into the beaker and 1700 ml. of electrolyte are added to the beaker. The effective volume of electrolyte contained within the cylindrical cathode is about 1115 ml. A x 1" workpiece having a total surface area of about .01 square foot, is secured to a work bar which is lowered into the electrolyte. This workpiece is a zinc alloy known by the trade name Zamak-3 (trademark of The New Jersey Zinc Company) and having the following composition as specified in the 1957 ASTM Supplement, Part 2, page 115:

Aluminum 39-43%. Magnesium 0.03-0.06%. Copper a- 0.10% maximum. Iron 0.075% maximum. Lead 0.005% maximum. Cadmium 0.004% maximum. Tin 0.002% maximum. Zinc remainder.

During the electropolishing, the electrolyte is agitated by means of work bar agitation, the work bar having a two inch displacement/one second stroke. A source of DC. current is connected to the workpiece and electropolishing is carried out for the desired length of time.

The following table, wherein are incorporated Examples 1 through 14, illustrates the effect of varying the electrolyte concentration. In these examples, the voltage range is 5.0-7.8 volts and the applied current is 2.5 amperes.

TABLE I Corn- Anode pocurrent Con- Ex, Bath ition, density. tact Temp Remarks No. grams; amns/ time, 1-. liter ft. min. aOH

1..-. 33 25 200 4 73-75 Light grey oxide coating, no cleaning or polishing.

2 34 50 200 4 73-75 Dark rey, blue oxide coating. no cleaning orpolishing.

3. 35 200 4 7345 Light grey oxide conttinc, no cleaning or polishing.

7..-- 39 500 200 4 73-76 Clean but not bright.

8 46B .650 00 4 7345 Clean and bright.

9."- 47B 600 200 4 73-75 Clean and very bright.

11. 79A-2 800 200 4 (loan but not bright,

pitted.

12 RDA-2 903 200 4 121 Do.

13--. Sin-2 1.003 200 4 Do.

14.-- 82A-2 1,190 200 4 149 Do.

1 In Examples 11-14 it was necessary to increase the temperature to the value shown in order to put the desired amount of NnfiH in solution.

As seen in the table, when the current density and temperature (except where indicated above) are kept constant, electropolishing does not take place until the composition of the electrolyte is about 450 g. of caustic soda/liter and that as the caustic soda concentration is increased, no polishing occurs above a concentration of 800 g./liter.

The following table incorporating Examples XV to XXIII, indicates the results obtained by varying the anode current density. In these examples, the voltage range is 611 volts and the contact time is 4 minutes.

TABLE II Corn- Anode posi current Ap- Ex. Both tion, density, plied Temp. Remarks N 0. grams] amps-J current, 1*. liter It. amps. NaOH 15. 46A 550 100 1. 5 73-75 Clean but not bright.

46B 550 200 2. 5 7345 Clean and bright. 46C 550 400 4. 5 73-75 D0. 16. 47A 600 100 1. 5 73-75 D0.

47B 609 200 2. 5 73-75 Clean and very bright. 47C 600 400 4. 5 73-75 Do. 17. 60A 665 100 1. 73-75 Clean and bright.

B 665 200 2.5 73 75 Clean and very bright. 60C 665 400 5. 0 75-75 Do.

18. T9A l 800 1.5 1 99 Clean but not bright,

pitted. 79A-2 800 200 2. 5 Do. 79A-3 800 400 4. 5 Do.

19- 80A1 003 100 1. 5 121 Do. 80A- 2 903 200 2. 5 121 Do. BOA-3 903 400 4. 5 123 D0.

20- 81A1 1. 003 100 1. 5 Clean but not bright.

81A-2 1, 003 200 2. 5 140 Do. 81A-3 1,003 400 4.5 D0.

21. S2A-l 1,100 100 1. 5 147 Not clean or bright. 82.42 1.190 200 2. 5 149 Clean but not bright. 82A-3 1. 190 400 4. 5 D0.

22-- 84H-1 665 600 NM 74 Clean and very bright.

84 iI-2 605 1000 N M 74 Do.

23-- 112A-4 665 800 NM 74-75 Do.

1 In Examples 18-21 it was necessary to increase the temperature to the value shown, in order to put the desired amount oi NaOII in solution.

From this table, it is seen that when using current densities of less than 200 amps/square foot, no bright polishing occurs until the composition of the electrolyte is at least 600 g. of caustic soda/liter and that above this concentration, the polishing obtained when using 100 amps/ square foot is not as good as that obtained when using a current density of 200 amps./ square foot. Additionally, it can be seen that current densities greater than 200 amps/square foot do not produce results which are appreciably superior to those obtained when using 200 amps/square foot, so that no advantage is obtained in using higher current densities.

In the following table, incorporating Examples 2445, the effect of varying the temperature of the electrolyte can be seen. In these examples, the voltage is 4.8 volts, the applied current is 2.5 amperes, the anode current density is 200 amps/square foot and the contact time is 4 minutes.

TABLE III Com- pe Ex. Both tion, Temp, Remarks No. grains] I liter NaOII 2 665 76 Clean and very bright.

2 605 97-99 Clean and bright.

-12 665 110 Clean but notbriglit:,liglit brown smut.

2 655 134 Clean but not bright, light grey coating.

2 665 152 Light grey coating.

-22 605 Dark grey coating.

79A-2 800 110 Clean but not bright, pitted.

013-2 800 129 Clean and bright, slightly pitted. 9D-2 800 154 Dark grey oxide coating.

.44.... 7913-2 800 178 Do. 35.... 79F-2 800 201 D0. 30.... Stilt-2 003 121 Clean and bright, pitting. 37.. 80B2 903 162 Clean and bright, light brown film. 38.... 806-2 903 171 Dark brown coating. 39.... 8OD-2 903 203 Dull bright grey oxide coating. 40,... 81.4-2 1,003 140 Clean but not bright. 41.... 818-2 1.003 174 Dark grey oxide coating. 42-... 810-2 1,003 205 Do. 4. 82A2 1,100 149 Clean but not bright. 44.... 82B-2 1,190 179 Clean but not bright,light grey smut. 45.... 82C-2 1,190 108 Dark grey coating.

From this table it is seen that with electrolyte concentrations of 665 g. of caustic soda/liter, no brightening of the workpiece occurs at temperatures above 99 F., while no cleaning and smoothing occurs at temperatures in excess of 134 F. Likewise, at higher concentrations of caustic soda, there is some cleaning and brightening at higher temperatures, but even at these higher concentrations no brightening occurs above about 160 F. and there is no cleaning above about 180 F.

The following table, incorporating Examples 46-62, illustrates the effects obtained by incorporating zinc oxide and other metallic oxides in the caustic soda electrolyte composition. In these examples, the voltage range is 4.8-6.2 volts, the temperature varied between 73-75 F. and the contact time is 4 minutes.

TABLE IV Composition, grams/liter Anode Bath current Applied Ex. No. density, current, Remarks NaOH ZnO Other anrips/ amps.

46.... 40 25 10 200 2. 5 Light grey oxide coating, no cleaning or polishing. 47.. 45A 50 10 200 2. 5 Dark gre -blue oxide coating, no cleaning or polishing. 48. 44A 75 200 2. 5 Do. 49.--. 41 100 200 2.5 Do. 50.... 45A 100 200 2.5 Light grey oxide coating, no cleaning or n polishing. 51.-.. 42 000 200 2. 5 Partial cleaning but no polishing. 5'1-.. 48A 400 100 1.25 Do. 488 400 200 2. 5 Do. 480 400 400 4. 5 Do. 53.-. 49A 500 100 1. 25 Do. 49B 500 200 2. 5 Do. 490 500 400 4. 5 Do. 54.... 50A 500 100 1.25 Cleaning and slight polishing.

50B 500 200 2. 5 Do. 500 500 400 4. 5 Do. 55.... 51A 600 100 1. 25 Clean and bright. 51B 000 200 2. 5 Do. 51C 000 400 4. 5 Do. 56- 52A 600 100 1. 25 Do. 52B (100 200 2. 5 Do. 526 600 400 4. 5 Do. 57-... 53A 625 100 1. 25 Do.

53B 625 200 2. 5 Clean and very bright. 53C 625 400 4. 5 Do. 58.. 54A 665 100 1. 25 Clean and bright.

MB 665 200 2. 5 Clean and very bright. 54C 665 400 4. 5 Do.

TABLE IVContinued Composition, grams/liter Anode Bath current Applied Remarks Ex No. density, current,

NaOH ZnO Other snaps] amps.

59-... 67A 605 AlzOa-lOiL... 190 1.25 Clean but not bright.

67B 665 AlzOs-100 200 2.5 Clean and bright. 67C 685 Al os-100-. 400 4.5 Do. 60--.. 68A 665 AlzQ325 100 1.25 Clean but not bright.

68B 665 Al oa-25... 200 2.5 Clean and bright. 68C 865 Al2OK25 400 4.5 Do. 61.-" 69A 665 Mg-25 100 1. 25 Clean but not bright.

69B 665 MgO-25.... 200 25 Clean and bright. 090 005 Mg025 400 4.5 Do. 62.--. 70A 665 100 1. 25 Clean and slightly bright.

70B 665 200 2. Clean and very bright. 70C 665 400 4.5 Do.

By comparing the above results with those in Table I, TABLE VI it is seen that there is no appreciable difference between electropolishing with and without zinc oxide or some other Applied 553? oxide in the caustic soda electrolyte. In either case, elec- 20 Ex Bath No. current, amps/ft." Remarks tropolishing does not begin until the critical concentration 3x3 53 135? of about 500 g. of caustic soda/liter is reached. Hence, although oxldes. do not appqar to adversely the 58".. 3202-21 0.1 1-7 Grey dull surface over full electropohshing, the1r presence in the electrolyte is apcurrent density range of anel. P y supeijfiuous- 25 69".. 3202-13 0.5 .s-25 DQ;

The following table, incorporating Examples 63-67, 70--.. 3202-0 1 1 4 Grey dull surface. shows the results obtained in determining the feasibility of 4.00 fial l zfi ll gl r fii gt carrying out electropolishing at a lower current density 71".- 3202-D 2 1-35 Do. over a longer period of time. In these examples, the 5 f but voltage range is 5.2-5.8 volts. 3O 3 g g i with na a ceaningun crneath. From the table, it is seen that no cleaning occurs until H50 Clea}, and Smooth but not a current density of at least 40 amps./ square foot 18 used brlght.

. 150-110 Clean and smooth, semiand that no polishing occurs until a current density of at bright; least 60 amps. per square foot is used, but in either in- 3202i 5 1-50 lzle u jnghiiideiilla tli stance, the time required is 45-60 minutes. Thus, al- 35 5on5 Clgainhtand smooth, serni- I'g thou gh electropohshlng can be carried out at current "H00 bright electroponshed densities below that of the preferred 200 amps/square 320241 gg g n fi m, clean unfoot the time required for such an operation would not 50-180 Olga n t i Smooth, Semi.

mg mal te 1t economically feasible and additionally larger 0 18mm very bright clecmponshei equipment would be required. 4

TABLE V Composition, Anode Bath grams/liter current Contact Tenllp, Ex. No. density time, g Remarks ampsj niin. NaOH ZnO it? 63 m l 665 100 10 5 7 Grey oxide coating, no cleaning or polishing.

120A-2 665 100 10 75 D0. 120A-3 665 100 10 30 75 D0. 12011-4 665 100 10 45 75 Do. e4 12013-1 665 100 5 76 Do. 12013-2 065 100 2c 15 76 Do. 1208-3 065 100 20 70 Do. 12013-4 665 100 20 45 70 Do. 65 1200-1 665 109 4G 5 78 Clean but not polished.

1200-2 665 100 15 78 D0. 120(3-3 am 100 40 30 78 D0. 120(1-4 one 100 40 7 D0. G6 120D-1 665 100 so 5 31 D0.

IZOD-2 665 100 15 81 Clean and slightly polished. 120D-3 065 6O s0 81 Do. D-4 665 100 60 45 81 Clean and bright. 67 120E-1 655 100 100 5 82 Clean but not poliShBd.

HOE-2 665 100 100 15 82 (lean and slightly polished. HOE-3 665 104) 30 82 Clean and bright. l20E-4 6G5 100 100 45 82 Clean and very bright.

The results in the following table, incorporating Examples 68-74, were obtained using a Hull cell. 267 ml. of electrolyte, containing 670 g. of sodium hydroxide/liter and 93 g. zinc oxide/ liter are placed in the Hull cell and electropolishing carried out for a period of 4 minutes at each of the applied currents indicated. The temperature of the electrolyte varied between 68 and 92 F. The panels used are 2V2" x 4" pure zinc sheets.

The above results indicate that pure zinc as well as zinc alloys can be elcctropolished by the method of the present invention. Additionally, the above results serve as a check on the results obtained in previous tables, indicating that a current density of about 50 amps./ square foot is the critical point at which electropolishing begins 75 to take place.

11 Example 75 To illustrate the build-up of zinc oxide concentration during electropolishing, a life test is made using as the electrolyte liters of caustic soda solution in a hard rubber tank containing 668 g. of caustic soda/liter and 0.2 g./liter polyoxyethylene ester of mixed fatty and rosin acid (Renex 20) as a defoamer. During this test the anode current density is 200 amps/square foot, the cathode current density is 13.8 amps/square foot and 12 oxide, electropolishing is carried out in 31.5 liters of electrolyte containing 665 g. of sodium hydroxide/liter, 100 g. of zinc oxide/liter, 0.2 g. of defoaming agent/liter (Renex 20). The anode current density is 200 amps/square foot, the cathode current density 7.4 amps/square foot and the solution density is 1.27 amps/liter. The electropolishing is carried out in a 32 liter tank of heat-treated steel, which tank forms the cathode and has a surface area of 5.4 square foot. As

the solution densit is 1.6 am s./liter. The anodes used are 1/2 inch Stripsyof alloy having a total sub the anode, .2 strips of Zamak-3 are used, having a total face area of .04/square foot, while the cathode is two surface area of sqllare AS m Example the sheets, 7 inches x 7 inches of heat-treated iron having anodes are electropohshed they are about a surface area of 0.58/ square foot. The temperature of consumed, whereupon they l replaced The condltlons the electrolyte is maintained within the range of about 15 of operation and results Obtamed are reported below in 7595 F., the average temperature being about F. The anode is electropolished continuously until it is about 75% consumed whereupon it is replaced with a new anode. The following table is a summary of tabular summary, the results being reported every 1020 hours. It should be noted that observations made during the intervals between the reported data indicate no significant deviation from the general trend.

TABLE VIII Cumulative Analysis, g.1. Bath Anode loss Anode No. of Zn in effi- Remarks Hrs. Amp. hrs/l. Ftfi/l. Free Zinc grams, Cnciency,

NaOH mulative percent 86.- 0.7 .86 .06 596 101. 8 17. 5 53 Anode clean, smooth and lustrous.

21. 2 26. 8 2. 0 685 108 450. 0 40 D0. 45. 1 57. 3 4. 3 548 120 958. 0 42 Do. 68. 3 87. 0 6. 45 509 137 1427. 0 47 D0. 84. 0 106. 0 7. 68 428 153 1, 746. 5 44 Do. 98. 8 125 9. 29 477 160 2, 038. 0 40 Do. 114.8 143 10. 61 484 177 2, 280. 5 29 Ron/103316511. of bath and replace with 1.5 1. 01 665 g. 1. a 122.0 150 11.11 508 173 2, 367.0 28 Anode clean, smooth and lustrous. 131. 2 160 11. 512 160 2, 461. 5 29 D0: 154. 4 185 13. 8 473 188 2,766. 5 31 Anode clean, smooth and semibright. 176. 9 215 15. 9 472 190 3, 201.0 39 Do. 198. 9 245 18. 2 388 195 3, 548. 0 34 Do. 215. 3 265 19. 7 392 202 3,817. 0 32 Anodeclean, smooth and not bright. 236. 9 294 21. 8 388 212 4, 153. 5 29 Do.

operating conditions and results obtained over a run of about 740 hours. These results are reported as taken By comparing the results of Example 76 with those in Example 75, it is seen, that as would be expected, the

every H shmld be noted that bservatlqns 45 time required for the zinc oxide content of the electrolyte made during the intervals between the reported data rm to buildm to the toleration Oint is about h If dicate no significant deviation from the general trend. 1 p h b h h h p It should be further noted that when the solution has as m t e at does notconfam i Pxlde reached h f th l h anode emcigncies are dropas compared to that which does contain Zll'lC oxide in the ping rapidly. 50 amount of g./liter.

TABLE VII Cumulative Analysis, g./l.

Anode loss Anode Bath of Zn in Etfi- Remarks N 0. Amp. Free grams, eieney,

Hrs. hrs/1. FtJ/l. NaOH'. Zine cumulative percent 29 8.5 13. 6 1.02 657 23. 4 41. 2 50 Anode clean, smooth and bright.

30. 3 44.4 3.62 622 41. 2 141. 5 50 Do. 62.5 100 7. 4 596 63. 2 302. 5 49 Do. 93. 5 149 11.2 533 451. 5 59 Do. 125. 8 201 15. 1 522 586. 0 42 D0. 158. 3 253 18. 0 489 152 718. 0 46 Anode clean and bright. 189. 8 303 22. 8 465 177 848. 0 40 D0. 4 216. 7 348 26.2 439 193 926. 5 24 Anode clean and semlbright. 255. 1 409 30.8 425 204 1,017. 5 32 Rarngfid 25% (by volume) 0! the bath and replaced with 1.251. of 670 g./l.

B, 262. 8 421 31. 6 476 155 1, 047.0 40 Anode clean, smooth and bright. 291. 3 467 35. 1 425 1,150.5 33 Do. 302. 2 484 36. 4 436 198 1, 187.5 44 Removed 25% of bath and replaced with 1.25 l. of 670 g. [1. NeOlI. 307. 9 493 37. 0 508 141 1, 208. 5 44 Anode clean, smooth and bright. 338. 6 543 40.8 496 172 1, 313. 0 35 Do. 366. 9 582 43. 7 448 186 1, 395.0 33 Anode clean, smooth and semlbrlgh t. 348. 3 626 47. 0 338 204 1, 490. 0 36 Do. 428. 2 668 50. 2 368 212 1, 558.0 25 Anode clean, hlueblaek oxide coated. Replenlshed 40% 0! bath with 2 l.

of 670 g./l. NaOI-I. 527 737 55. 2 437 156 1, 716. 7 37 Anode clean, smooth and bright. 567 793 59. 6 382 171 1, 824. 5 32 D0. 608 851 64. 2 362 1, 918.2 25 Anode clean, smooth and sernibright. 681 953 71. 6 362 202 2,028.6 12 Anode clean, smooth and not bright. 736 1,029 77. 2 320 212 2,082.0 7. 2 Anode clean but dull, oxide coat/ed.

Example 76 Example 77 In order to show the zinc oxide build-up during elec- To show the effect of daily replenishment of electrolyte tropolishing in an electrolyte which initially contains zinc 75 to simulate drag-out with regard to the zinc oxide builds,oss,sse

13 up, a life test is made using liters of electrolyte containing 669.8 g. of sodium hydroxide/liter, 93.6 g. of zinc oxide/liter and 0.2 g./liter defoamer (Renex The anode current density is 200 amps./ square foot, the cathode current density 13.8 amps./ square foot and the solution density is 1.6 amps/liter. The temperature is maintained within the range of about 70-90 F. with the average temperature being about 80 F. The cathode is formed from two 7 inch x 7 inch sheets of heat-treated steel having a total surface area of .58 square foot. The anode is a /2 inch strip of Zamak-3 alloy having a total surface area of .04 square foot. 5% or 250 ml. of the electrolyte is removed daily and replenished with a like amount of fresh caustic solution (670 g. NaOH/ liter). As in the two previous exrnples, the anode is electropolished until it is about 75% consumed, whereupon it is replaced. The operating conditions and results obtained over a period of about 700 hours are shown below in a tubular summary. It should be noted that observations made during replacing it with fresh caustic soda (670 g./liter), even though containing about 95 g./liter of zinc oxide at the beginning, at the expiration of 700 hours operation the zinc oxide concentration is still considerably below the toleration point. Examination of the anodes for the run and the various test specimen done at intervals show a clear, smooth and very lustrous zinc surface. Even at 700 hours, it is still electropolishing satisfactory.

The following examples show the results obtained in testing different materials as cathodes for use in the present process. The half-tones which form the illustrative figures of the present invention were made of the cathode before and after electropolishin In all of these examples, electropolishing is carried out in a one liter beaker containing 784 ml. of electrolyte, containing 632 g./liter sodium hydroxide and 1 01 g./liter zinc oxide. The anode is of Zamak-3 and has an area of 0.052 square foot. The anode current density is 200 amps/square foot and the solution density isl.

TABLE X Cathode Voltage Temp. Current Ex. C athode material area, It. average average, etficieucy. Remarks F. percent rum steel, heat-treated 0.156 5.0 83 47 N0 plating of Zn, I'lz gassing at cathode, very bright polish on anode. Stainless steel, heat treated--. 0. M3 5. 0 7 46. 4 Do. Magnetite (L 095 4. 7 84 50. 4 Do, Cast iron, HCI cleaned.. t). 073 4.; S0 55. 2 Do. Drum steel.... U. 156 4. 3 84 54. 2 Zn plate on cathode, no Il gassing at cathode,

very bright polish on anode. Stainless steel... 0.196 5. 0 82 46 Do. F9203 coated Stet. 0.1516 5. 2 82 50.3 Do. Pure zinc 0.156 4. 9 86 U) DO. Graphite... 0.1% 5.0 82 45. 6 Do. Galvanized s e. 0.156 4. 8 82 51. 6 Do. ZamalrrS" 0.136 4. 8 80 48. 7 D0. Nickel. (l. 226 4. 6 82 53. 1 DO. Lend-.- 0.198 5. (I 80 47. R Do. Copper" (1.153 5.0 83 a9. 4 Do. Tungsten 0. 087 5.0 80 50. 2 Do. Lead oudi 0.198 4. 6 8t] 50. 3 110. Copper oxidized 0.153 5. O 84 59.1 lJo. Tungsten OXldlZlKl... 0. (185 2. 5 76 68.4 Do. Platinum. 0. 045 6.0 80 49. 4 Do. Titanium. 0.181 4. 9 7 48. it Do. Zirconium 0. 068 5. 4 89 4S. 3 Do. Silver 0.136 5.0 81 50. 2 Do. Molybdenum- 0. 068 5. 0 79 51. 3 Do. Alumi|1um 0.198 4. B 81 5(1. 6 Do. Nickel plated copper... 0.138 4. 2 T7 51. 2 Do, Chromium plated copper 0. 138 4. 1 78 45. 3 Do.

1 Not, measured.

the intervals between the reported data indicate no significant change from the general trend.

TABLE IX Cumulative Analysis, g/l. Anode Bath loss of In g f u A 11 ll It m r o z o i g- Olen ours in rs. i re. 11 Hill 4 p NaOH tlve rum 21 34 2. 55 580 106 91 02 36 57 4.27 1'49 103 1(0 42 60 77 5. 90 556 1.05 208 43 72 114 8. 55 533 117 294 43 106 170 12. 5-1 125 4'27 128 213 15. 90 544 122 48 143 234 17. 50 544 125 5H] 50 176 279 20. 90 528 124 698 -12 190 199 22. 4 533 122 753 49 244 352 26. 4 475 117 S31 43 301 422 31.3 120 1,101 357 500 37. 6 476 119 1. 303 43 422 591 44. 4 470 1'21 1, 550 i 436 63 51. 2 472 124 1. T88 14 535 751 56. 4 48'! 1'29 1, 973 41 619 857 65. Q 476 2, 273 43 679 71. 2 455 124 2, 505 44 From the above, it is seen that where the electrolyte is replenished daily by removing about 5 of its volume and From the above, it is seen that only when the cathode has a surface which contains Fe O is there no plating of zinc on the cathode. With all of the other materials used, zinc plates on the cathode, and there is no evolution of hydrogen gas at the cathode.

In substantiating the visual observation of the above examples with experimental data, the following examples, which are carried out separately, are given in which the oxygen and hydrogen efilciencies during electropolishing using various cathode materials are measured. In these examples, the anode is Zamak-S and has an area of 0.65 square inch. The anode current density is 200 amps/ square foot. The electrolyte is 750 ml. containing 632 g. sodiiun hydroxide/liter and 101 g. zinc oxide/liter. The applied current is 1 amp. and the solution density is 1.25 amps/liter.

From the table it can be seen that when using the cathode having a surface which contains Fe O on which cathode there is no plating of zinc, the hydrogen current efliciencies are very high showing that in these instances electrolysis of the water in the electrolyte is taking place rather than plating of the zinc on the cathode.

aoaaese 1 10 TABLE Xi Anode Cathode 9111- Oz Gas Ili Gas Ex. Cathode material area Time Volt. Temp. cieney eili- Elh- Remarks inches 2 min. ave. ave, F (weight ciency, ei'eney, loss), percent percent percent Drum. steel 18 240 4. 25 81 59. 33.1 1. 3 Spongy Zn plated on cathode. /10 steel, pickled. 15. 3 240 5. 7 70 53. 0 125. 5 2. 0 Do. Graphlte 9 117 5. 3 78 G2. 8 41.1 6. 6 Do. Zirconium 11 151 5. 4 81 67.1 4%. 3 l2. 0 D0. Stainless st 15. 9 180 5.2 82 00. 5 31 15 Do. Copper, oxidized 18 89 5.8 78 T0. 4 33. 3 15. 2 Do. Nickel a. 16. 5 172 5. 2 81 71. 0 36.1 18 Do. Molybdenum .l 11. 4 150 0. 2 70 65. 7 40. 5 24. 4 D0. Cast iron, pickled 9. 4 203 5. 6 70 58. 4 30 52.5 No Zn plate on cathode. l13 Drum steel, heat treated... 18 188 5. t 02 50. 2 45. l 81. 2 D0. 11L... Stainless steel, heat treated 11. 3 133 5. 5 75 48. 8 45. 5 87. 5 Do.

To attempt to explain the behavior of cathodes having a surface of l e- 0 potential measurements are carried out with several cathode materials in both 45% caustic soda solution and sodium zincate (665 g./liter NaOH, 100 g./liter ZnO) media. The measurements are made using the conventional direct method" for electrode potential and over potentials under continuous electrolysis in a 1000 ml. solution with constant stirring.

Both the reference and the elccti'opolisliing cells are kept at a constant temperature of C.-* :O.1. The anode area is kept constant at 3.9 cm. while the cathode area varies from 50-100 cm. The anode material is Zamak-3 alloy. Because of the alkali medium, the reference electrode selected is mercury-mercuric oxide; it is checked with a standard saturated calomel electrode (0.2415 v.).

Table XIIA gives the reference electrode potentials for various sodium hydroxide concentrations. The values (unknown in literature) are reported to serve as the basis for the measurements. The E.M.F. of the mercurymercuric oxide electrode with the zincate solution is determined to be 0.065 v. (not included in Table XII-A).

Table XII-B gives the values obtained with various cathodes under various current densities in both the 45% caustic soda and zincate media. FIGS. 25 and 26 give a graphic comparison of these potentials.

TABLE XIIA Potentials of the Reference Electrode With Difierent Concentrations Alkali Plus. 25 and 26 illustrate the electrode potentials for the various metal electrodes in two different media, along with the cell voltage obtained. In going from NaOH solution to zincate solution, the overvoltage of the zinc electrode changed; the iron electrode in the zincate media shifted rapidly toward the zinc potential as it becomes coated with zinc metal. Similar results were obtained with other metals; when suificient zinc is deposited the potential of the respective metal (e.g. nickel) tends to shift toward the zinc electrode potential. Regardlng the heattreated iron containg cathodes, particularly the heat-treated (Fe O coated) steel, it can be seen from FIGS. 25 and 26 that its potential remains essentially the same, and maintains the same relative position and its overvoltage is almost constant regardless of whether the polishing is carried out in straight caustic soda solution or in the presence of high concentrations of zinc. Only minor changes in the electrode potential (a few millivolts) may occur depending on the conditions of heat treatment. This is also true for all current densities, as hydrogen is deposited on such cathodes, the supply bcing maintained by the dissociation of water into H and OH ions.

It is apparent that current density and the voltage are very related in electropolishing of metals. The cell potential curves given above are very similar in shape to the anodic potential curves (not included here); as the current is increased, there is a minor increase in voltage until a plateau is reached where the voltage rapidly incrcases with little increase in current density.

NEHQ Average potential Avmge potmfldl Thereafter, the increasean voltages are fairly uniform (Normality) IIgI'IgO BDII aOH 111111110 Elecas the current density is increased. The polarization is. b. 0 r0 becomes negative, possibly indicating that oxygen is 0 97 +0 1280 0 11% evolved more readily than on a reversible electrode. ass +0I1420 0:0!l 80 It is believed that the oxide film is formed by the F o I I T8322 electrochemical oxidation of the anodes until the point 10.30 +0.1749 -0.006e where a break-through occurs and thereafter electropol- V0 u I v 23 1811233 $1835: ishing 15 produced. The point on the anodic potential 9- +0.18% 53 curve is at about 40-50 amps/square foot which is in agreement with the result obtained by varying the cur- TABLE XII-B Electrode potential in 45% NaOH Electrode potential in the zineiite solution Current density, Stainless Steel Stainless Nickel Steel Steel 7 malcm. Platinum Cast steel heat Cast Steel steel 3 Z nc heat Steel zinc Linc (rough) iron heat treated magnetite 10/10 316 bright bright treated 10/10 coated treated bright 0 0. 994 1.003 1 002 1.023 1.007 1.037 1 009 1.105 1 473 1.030 1.103 1.307 1.303 0. 1.007 1.013 037 1.100 1.023 1.153 200 1.202 1 600 1.050 1.300 1.373 1.377 0. 1.013 1.023 053 1.113 1.011 1.100 223 1.220 1 000 1 032 1.332 1.332 1. 381 0. 1.022 1.032 005 1.113 1.030 1.101 235 1. 230 1 744 1 0117 1.333 1.333 1.333 0 1.023 1.010 072 1.121 1.030 1.103 245 1.21s 1 702 1 033 1.334 1.3515 1. 330 1. 1.034 1.047 077 1.127 1.007 1. 203 251 1. 257 1 77:3 1 090 1.335 1.357 1.380 1.5 i, 1. 1 s 1 093 1.333 1.390 1.304 2.0 1. 1. 1. 1 315 1 090 1.390 1.394 1.400 2.5 1. 1. 1. 1 329 1 030 1.392 1.303 1 405 3.0 1. 1. 1.' 1 811 1 102 1.303 1.397 1.410 4.0 1. 1. 1. 1 858 1 107 1.301 1.303 1.413 5.0 1. 1. 1. 1 370 1 110 1.393 1.400 1.120 0.0 1. 1 37s 1 113 1.330 1.100 1.134

17 rent density (Table II) and the Hull cell test panel (Table VI).

While there have been described various embodiments of the invention, the methods described are not intended to be understood as limiting the scope of the invention, and it is realized that changes therewithin are possible, and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method of anodically electropolishing an article having a surface of metal selected from the group consisting of zinc and zinc base alloys, which comprises making the article to be polished the anode in an electrolytic cell, the electrolyte of which is comprised of an alkali metal hydroxide in the amount between about 450 and 900 grams/liter, maintained at a temperature between about 60 and 220 F. and passing an electric current through said electrolyte between said anode and a cathode having a surface of Fe O said current being passed at an anode current density between about 50 and 80D amps/square foot for a period of time sufiicient to electropolish said anode, and using a voltage sufficient to provide this current density, said voltage being below that at which zinc will be electrodeposited on said cathode having a surface of Fe O 2. A method of anodically electropolishing an article having a surface of metal selected from the group consisting of zinc and zinc base alloys, which comprises making the article to be polished the anode in an electrolytic cell, the electrolyte of which is comprised of an alkali metal hydroxide in the amount between 572 and 763 grams/ liter, maintaining said electrolyte at a temperature between about 70 and 90 F., passing an electric current through said electrolyte between said anode and a cathode at an anode current density of about 200 amps/square foot, said cathode having a surface of Fe the passage of current being for a period of time sufficient to electropolish said anode, and using a voltage sufficient to provide the above current density, said voltage being below that at which zinc will be electrodeposited on said cathode having a surface of Fe O 3. The method as claimed in claim 2, wherein the alkali metal hydroxide in the electrolyte is sodium hydroxide.

4. The method as claimed in claim 3, wherein the solution current density used does not exceed 2.5 amps./ liter.

5. The method as claimed in claim 3, wherein the electrolyte contains zinc oxide in an amount up to about 175 grams/liter.

6. The method as claimed in claim 3, wherein a defoaming agent is added to the electrolyte.

7. The method as claimed in claim 5, wherein a defoaming agent is added to the electrolyte.

8. A method of anodically electropolishing an article having a surface of metal selected from the group consisting of zinc and zinc base alloys, which comprises making the article to be polished the anode in an electrolytic cell, the electrolyte of which is comprised of sodium hydroxide in the amount of about 665 grams/liter, maintained at a temperature of about 76 F. and passing an electric current through said electrolyte between said anode and a cathode at an anode current density of about 200 amps./ square foot, said cathode having a surface of Pe o the passage of current being for a period of time suflicient to electropolish said anode, and using a voltage sufficient to provide the above current density, said voltage being below that at which zinc will be electrodeposited on said cathode having a surface of Fe O 9. The method as claimed in claim 7, wherein the electrolyte contains zinc oxide in an amount up to about grams/liter.

10. The method as claimed in claim 8, wherein a defoaming agent is added to the electrolyte.

11. The method as claimed in claim 9, wherein a defoaming agent is added to the electrolyte.

12. The method as claimed in claim 8, wherein the solution current density is maintained below 2.5 amps! liter.

13. The method as claimed in claim I, wherein the cathode is made of hot rolled steel.

14. The method as claimed in claim 2, wherein the cathode is made of hot rolled steel.

15. The method as claimed in claim 8, wherein the cathode is made of hot rolled steel.

References Cited in the file of this patent UNITED STATES PATENTS 670,201 Kendall Mar. 19, 1901 931,513 Specketer Aug. 17, 1909 2,655,472 Hilliard Oct. 13, 1953 OTHER REFERENCES Nature, vol. 142, 1938, pp. 4778, 1161.

Chemical Abstracts, 1942, vol. 36, p. 6919.

Canadian Journal of Chemistry, vol. 31, 1953, pp. 422-438. 

1. A METHOD OF ANODICALLY ELECTROPOLISING AN ARTICLE HAVING A SURFACE OF METAL SELECTED FROM THE GROUP CONSISTING OF ZINC AND ZINC BASE ALLOYS, WHICH COMPRISES MAKING THE ARTICLE TO BE POLISHED THE ANODE IN AN ELECTROLYTIC CELL, THE ELECTROLYTE OF WHICH IS COMPROSED OF AN ALKALI METAL HYDROXIDE IN THE AMOUNT BETWEEN ABOUT 450 AND 900 GRAMS/LITER, MAINTAINED AT A TEMPRATURE BETWEEN ABOUT 60* AND 220** F. AND PASSING AN ELECTRIC CURRENT THROUGH SAID ELECTROLYTE BETWEEN SAID ANODE AND A CATHODE HAVING A SURFACE OF FE3O4, SAID CURRENT BEING PASSED AT AN ANODE CURRENT DENSITY BETWEEN ABOUT 50 AND 800 AMPS./SQUARE FOOT FOR A PERIOD OF TIME SUFFICIENT TO ELECTROPLISH SAID ANODE, AND USING A VOLTAGE SUFFICIENT TO PROVIDE THIS CURRENT DENSITY, SAID VOLTAGE BEING BELOW THAT AT WHICH ZINC WELL BE ELECTRODEPOSITED ON AID CATHODE HAVING A SURFACE OF FE3O4. 